Load measuring devices and cells are known in the art. For example, Gallo, U.S. Pat. No. 4,043,190, discloses a meter for measuring mass or force wherein the sensed displacement acts indirectly on the tension of the two transversely vibrating electrically excited strings. Sette et al, U.S. Pat. No. 4,170,270, disclose an apparatus for preventing the overload of a load cell used to measure deflection. Blawert et al, U.S. Pat. No. 4,237,988, similarly disclose an overload protection device for precision scales. Paros, U.S. Pat. No. 4,384,495, discloses a mounting structure for double bar resonators to ensure symmetrical loading of the resonator responsive to external forces. Also, Paros, U.S. Pat. No. 4,751,849 discloses various mounting structures for use with force sensitive resonators.
Further, Streater et al, U.S. Pat. No. 3,712,395, disclose a weight sensing cell which includes two differentially loaded vibrating members. Suzuki et al, U.S. Pat. No. 4,196,784, disclose a weighing scale having an interior load cell. Great Britain Pat. No. 1,322,871 discloses a force measuring apparatus having a pretension string which is excited to a state of transverse oscillation by an electronic circuit. Gallo, U.S. Pat. No. 4,300,648, also discloses a meter for sensing mass and force comprising two flat springs lying in a parallel plane. Pulvari, U.S. Pat. No. 3,274,828, discloses a force sensor based on piezoelectric oscillators.
Also, Reid et al, U.S. Pat. No. 3,366,191, disclose a weighing apparatus which relies on a bridge circuit. Norris, U.S. Pat. No. 3,479,536, discloses a piezoelectric force transducer which is a piezoelectric vibratory beam mounted to receive compressive and tensile forces along its length. Agar, U.S. Pat. No. 3,529,470, discloses a force transducer having a composite strut with two bars which are to be maintained in transverse vibration at a common resonance frequency by electrical feedback wherein the frequency of vibration indicates the force applied to the composite strut. Corbett, U.S. Pat. No. 3,541,849, discloses an oscillating crystal force transducer. Wirth et al, U.S. Pat. No. 3,621,713, disclose an instrument for measuring masses and forces which when stressed by a load shows variation in frequency. Saner, U.S. Pat. No. 3,724,572, Van de Vaart et al, U.S. Pat. No. 3,853,497, Melcher et al, U.S. Pat. No. 3,885,427, and Paelian, U.S. Pat. No. 3,915,248, all disclose a weighing system which functions by force or weight being transmitted to frequency sensitive elements. Meier, U.S. Pat. No. 3,963,082, Wirth et al, U.S. Pat. No. 4,088,014, Jacobson, U.S. Pat. No. 4,143,727, Ebbinge, U.S. Pat. No. 4,179,004, all disclose force sensing load cell.
Finally, Eer Nisse, U.S. Pat. No. 4,215,570, discloses a miniature quartz resonator force transducer having the shape of a double ended tuning fork. Check et al, U.S. Pat. No. 4,239,088, disclose a scale with weight-to-period transducer which provides an oscillating output, the period of which varies as a function of the weight to be measured. Ueda et al, U.S. Pat. No. 4,299,122, disclose a force transducer based on a vibrator having a pair of plate-shaped vibrating pieces parallel with each other. Paros et al, U.S. Pat. No. 4,321,500, disclose a longitudinal isolation system. Eer Nisse et al, U.S. Pat. No. 4,372,173, disclose a resonator force transducer which includes a pair of elongate generally parallel bars coupled at their ends with a double ended tuning fork arrangement.
Recently, quartz double-ended tuning forks have been used as force sensors in environments where the tension resisted the movement of the loaded structure, or the tension was produced by strain within the loaded structure. Levered systems and parallel guiding structures have been used where the force applied to the force sensing crystal was a fraction of applied load. The force sensing crystal was generally small since the force required to cause adequate frequency change in the resonant double-ended quartz tuning fork did not need to be great.
However, the loaded structure had to be massive to resist effects of undesirable lateral deflection. The flexing portions of these structures which acted as parallel bending beams or bending fulcrums carried some load since the force sensing crystal and its bonded joints deflected when tension was applied to the crystal.
The prior art load cells were dependent on the stability of the loaded structure and the bonding joints, over temperature and time, for output stability. For example, Albert, U.S. Pat. No. 4,838,369 discloses a load cell intended to provide a linear relationship between the signal generated and the force sensed. Albert uses a specific crystal design attached by screws to the frame of the load cell which creates a frictional joint resulting in inadequate zero return and cell precision. Albert relies on a longitudinally rigid structure to resist interferences from varying load positions. The load cell of Albert is designed so that force expended on the load cell, when stressed, results in work or energy loss within the screw joints. In turn, this phenomenon also results in poor zero return and precision. Without attention to material similarly, non-strain sensitive designs, and reduction or cancellation of creep and hysteresis, Albert cannot provide a load cell which truly negates material and temperature effects.
Generally, material aging in these apparatus often caused long term performance to suffer after calibration. Further, these apparatus were limited in resolution by the degree in which anelastic creep and strain hysteresis were compensated for in their design. The quartz crystal bonding joints would often compensate for creep and hysteresis caused by the loaded structure with their own counteracting creep and hysteresis. When the quartz crystals were bonded using adhesives such as epoxies, stresses were introduced in the glue joints and crystal because of differential expansion between the base and the quartz and epoxy shrinkage during curing.
Further, as these stresses relaxed over time, the characteristics of the bonded joint changed because of the nonlinear stress-strain curve of the adhesive. This caused the load cell to have excessive zero and span shift over time until the glue joint stresses had relaxed. Differential expansion between the quartz and the structural material would cause the force sensor to have an output due to the temperature as well as applied load.
As a result, a need exists for a load cell which can compensate for changes in modulus of elasticity, anelastic creep, and strain hysteresis occurring in the elements of the cell due to stresses created by the environment of application.